Conventional power cylinders have a single piston and a single rod. The piston is slidably mounted within a cylinder. The rod is fixed to one end of the piston and protrudes through one end of the cylinder. Pressurized hydraulic or pneumatic fluid is injected into the rod end of the cylinder and withdrawn from the opposite end to force the piston towards the opposite end, thereby retracting the rod within the cylinder. The direction of pressurized fluid flow is reversed to force the piston towards the rod end of the cylinder, thereby extending the rod from the cylinder.
Conventional power cylinders of the foregoing type are subject to a number of disadvantages. For example, the seals used in conventional power cylinders are subject to substantial wear and are generally the components which require maintenance most frequently. In particular, seals which are exposed to the external environment in which the cylinder operates may wear out more quickly. Also, forces imposed on the rod during normal use can bend the rod, accelerating wearing of the seals and/or rod support bearings.
The present invention reduces the wear on each seal, increases the number of seals, and protects the seals from the external environment. For example, seal wear is reduced by providing dual pistons mounted in separate cylinders. This reduces the length of rod travel relative to each seal. Typically, the reduction in travel is on the order of half that of a conventional cylinder.
In conventional power cylinders, an eye is often provided on the end of the rod for attachment to external machinery. This may cause a design problem, in that the rod diameter may have to be increased to provide adequate space for connecting the eye to the rod. Consequently, the rods used in conventional cylinders are sometimes larger and more expensive than is required to support the loads imposed on the cylinder by normal use. The present invention avoids this problem by attaching any necessary eyes to the cylinder rather than to the rod. Rods employed in the present invention therefore do not require an eye. This allows suitable rods to be selected based only on the linear load to be supported by the rod during normal use.
Another disadvantage of conventional cylinders is that a single rod and piston tend to rotate within the cylinder. If several such cylinders are connected in series (to form a robotic manipulator arm, for example) the rotational tendency of each cylinder may affect the stability of the structure and prevent accurate control thereof. Certain embodiments of the invention solve this problem by employing two or more parallel rods. Each rod passes through a separate aperture in the rod end of the cylinder. The rods are thus held in place relative to the cylinder and prevented from rotating as they extend and retract.
In embodiments of the invention having two or more rods, hydraulic fluid may be allowed to pass through the rods between first and second hydraulic cylinders. This simplifies the external hydraulic circuitry required to operate the cylinder and allows the present invention to be directly substituted for conventional cylinders without any alteration of the existing hydraulic circuitry associated with the conventional cylinder.